Cataplexy is a sudden and transient episode of loss of muscle tone, often triggered by emotions such as laughter, fear, anger, frustration, annoyance, nervousness, embarrassment, and sadness. The sudden loss of muscle tone in cataplexy is similar to rapid eye movement (REM)-associated muscle atonia during sleep, but it is occurring during wakefulness. A cataplectic attack is sudden in onset and is localized to a specific muscle group or parts of the body. The subject is lucid during this attack; consciousness is always maintained at the onset of cataplexy. A full-blown attack may occur and results in complete muscle paralysis with postural collapse and possible injury. However, most often patients with postural collapse have the capability to avoid injury because the fall is slow and progressive. The more commonly limited cataplectic attacks involve the head and face, neck, upper limb, and more rarely lower limb known as “knee buckling.” Attacks can last from a few seconds up to ten minutes, and may occur up to several times per week. In some patients, status cataplecticus, or periods of repetitive loss of muscle tone, occurs and can last for hours or days.
Cataplexy is a rare disease (prevalence of fewer than 5 per 10,000 in the community), but affects roughly 70% of people who have narcolepsy. However, in some cases, cataplexy occurs without the co-occurrence of narcolepsy. Furthermore, upwards of 30% of patients with narcolepsy may never experience cataplexy. The exact cause of cataplexy is unknown, but the condition is strongly linked to experiencing intense emotions and reduced levels of the neurotransmitter hypocretin. Cataplexy is considered secondary when it is due to specific lesions in the brain that cause a depletion of the hypocretin neurotransmitter. Cataplexy can also be present as a side effect of discontinuation of certain drugs, such as selective serotonin re-uptake inhibitor (SSRI) antidepressants.
Cataplexy, whether associated with narcolepsy or other causes, is disabling and potentially dangerous. Regardless of cause, cataplexy is linked to a variety of transportation and industrial accidents and cause decreased job performance and considerable subjective distress. A therapeutic agent that reduces or eliminates cataplexy would have important implications not only for individual patients, but also for public health and safety.